Physicians and surgeons have long needed and desired to have a process and apparatus for determining, displaying and recording accurate information concerning instantaneous rates of flow of blood, volume of blood which flows during one pulse (sometimes called stroke volume), the volume of blood which flows during a certain unit of time such as a second, a minute, etc. (sometimes called cardiac output), and other parameters of blood flow, without the necessity for invading the blood vessels.
The body section involved may be the chest which contains the heart and larger arteries, but other body sections such as a leg or an arm may also be utilized.
Because blood, being an electrolyte, is more conductive to electricity than is solid tissue such as muscles, bones or viscera, it has been suggested that the electrical impedance of the section is indicative of the proportion of blood to the tissue in the section. Kubicek et al provided an impedance plethysmograph by which the value of Z, the impedance of the section of the body under test, was determined on a continuing basis. See U.S. Pat. No. 3,340,867 and the article entitled "Applications of the Minnesota Impedance Cardiograph" by Kubicek et al, in publication entitled "Development and Evaluation of an Impedance Cardiographic System to Measure Cardiac Output and Other Cardiac Parameters" by W. G. Kubicek, published by University of Minnesota July 1, 1968-June 30, 1969. Kubicek's apparatus included an outer pair of electrodes one of which was affixed to the patient's neck and the other of which was affixed to the patient's waist and an oscillating current was passed between these outer electrodes. A second pair of electrodes was likewise affixed between the first pair of electrodes for the purpose of measuring the electrical impedance. Although Kubicek's apparatus provided a proper representative value for Z, it provided no means for instantaneous and continuous determination of the rate of flow or volume of flow during one pulse or heartbeat or during any selected period of time.
Mount, in his U.S. Pat. No. 3,835,840, disclosed a plethysmograph apparently of the type disclosed by Kubicek, by which the impedance was determined, and the impedance so determined was fed through a circuit including electronic devices to obtain a signal which was recorded and which was expected to represent the rate of blood flow. But the signal obtained from the apparatus as disclosed by Mount did not represent the rate of blood flow with such accuracy as would be required to make it of substantial value to physicians and surgeons.
Accordingly, I have set about to discover apparatus and processes which do properly yield a signal which accurately represents and indicates the rate of blood flow through a body section on an instantaneous and continuous basis, and which will also indicate stroke volume and the cardiac output.